The practical problem that illustrates the need for such a system is in the inspection of the physical connections between silicon integrated circuit "chips" and their mechanical carriers. These connections are made by thin wires (10-500 .mu.m) running between small solder pads on the wafer and pins in the casing. The positioning of these wires is critical and must be maintained within certain limits. At present this is done manually by human operators who visually inspect each chip and reject those that lie outside the acceptable limits. The ideal system would, however, comprise a relatively cheap mechanical apparatus that would give a simple pass or fail test to each item.
With this problem in mind a programme of experiments was devised to see if holography could be utilised as a tool in this type of inspection. Holography offers the capability of recording the spatial detail of an object from many different perspectives.
An approach was tried to see if the "test object" could be compared by super-imposition with a virtual image hologram of the "perfect object" on a microscopic level. Several problems were soon encountered:
(a) It was discovered that for any comparison to be possible, either a human operator or a very sophisticated image analysis device was required.
(b) When smaller (less than 0.5 mm) objects were examined microscopically the problem of laser speckle soon limited the optical resolution.
A second approach which is adopted by the present invention, utilizes a real focused image hologram of the "perfect object". The image is of the pseudoscopic form (i.e. inside-out and back-to-front) and allows a real "test object" to be positioned within the confines of the focused image formed in space by the hologram. By simply measuring, for example, the light scattered back by the real object, a measure of coincidence can be detected.
Before describing the system of the present invention in detail, a brief theoretical discussion will be given with reference to FIGS. 1 to 4 of the accompanying drawings.
In FIGS. 1 to 4, the components shown are as follows:
A-a laser
B-a beam splitter
L1 and L2-lenses
M1-a flat mirror
M2-a collimating mirror
O-an object
P-a plate
Q-back scattered light
H-a hologram
D-viewer or detector
R-collimated reference light.